Auto and I have put gearbox oil coolers on as its air cooled in-font of rad and new rad as I have looked and looked but I can’t see how in anyway water can get in the box but I am loosing the water from the expansion tank so apart from the head I don’t have a clue

Can anyone tell me how it is that after having a head gasket done on discovery 2 I keep getting water in the gearbox and loose drive and no it never did this before so think myself it must be bad job on head but can’t see where water or how it’s getting in to gearbox